1. Field of the Invention
This invention relates to ventilation systems, and particularly to a system for measuring the flow of air through commercial ventilation systems, including those with centrifugal fans.
2. Description of Related Art
Ventilation systems in many commercial settings are required, either by code or by functional specifications, to have a certain minimum flow rates. For example, in commercial buildings, a minimum level of air flow is required to maintain a healthy air quality within the building. Similarly, in other applications, such as clean rooms, a certain level of air flow must be maintained to allow adequate filtration and removal of airborne particulates.
To ensure that the air flow requirements for a particular system are met, it is advantageous to be able to precisely measure the rate of air flow through the system. Systems without precise flow measuring capability are frequently overdesigned to have excess capacity, and are frequently operated at excess levels to ensure compliance with operating specifications. This adds unnecessarily to the expense of both the systems and their operation.
In addition, filters used in some systems can become clogged with particulates, making them more resistant to air flow over time. This increased resistance may cause the air flow of the system to drop below acceptable levels unless it is precisely monitored and steps are taken to compensate for the increased resistance.
Unfortunately, many methods of measuring air flow in a ventilation system can dramatically decrease the efficiency of the system. The efficiency of a ventilation system is a measure of how readily air flows through the system or, conversely, the system's resistance to air flow. Each component of a system through which air flows presents a certain amount of resistance to air flow. This resistance is determined by the size and shape of the component, and by the nature of any obstacles or surfaces over which the air flows. Generally, components that are wider, smoother, straighter and shorter have less resistance to air flow, and therefore provide a more efficient system.
Improved efficiency can permit the use of a lower capacity fan to generate a given level of air flow in a system, and can require less energy to maintain a given level or air flow. In this manner, improving the efficiency of a ventilation system can reduce both the equipment and operating costs for the system. Many systems for measuring airflow, however, have just the opposite effect. Many flow measuring systems increase a system's resistance to air flow, and thereby reduce the system's efficiency.
Existing air flow measurement systems have had to balance the trade-offs between efficiency and precision of measurement. Existing measurement systems typically create an obstruction or constriction within the air flow, and measure the effect of the obstruction or constriction on the air pressure at a certain point in the system. Increasing the size of the obstruction or amount of the constriction generally increases the precision of the flow measurements but also increases the negative impact of the measuring system on the system efficiency.
A pitot tube measuring system is an obstruction-type measurement device. A typical pitot tube has an orifice facing directly upstream to provide a total pressure measurement and an orifice oriented to provide a static pressure measurement. From this information, the velocity of the air stream can be determined. The velocity multiplied by the cross sectional area at the point of the measurement equals the air flow.
The precision that can be obtained with a pitot tube measuring system generally depends on the size, number, and position of the openings in the air stream. To obtain accurate measurements, it is generally desirable to have several pitot tubes or orifices located at various positions within the air flow. Otherwise, local variations in the velocity of the flow may result in an erroneous reading. However, each pitot tube creates a disturbance in the air flow, thereby increasing turbulence and resistance and decreasing efficiency.
Pitot tube measuring systems are generally more accurate when positioned in a region with a smaller cross-sectional area and, hence, higher velocity air flow. Positioning a pitot tube measuring system in such a location, however, also increases the negative impact of the pitot tube on the efficiency of the system.
A venturi tube measuring system is an example of an air flow measurement system that operates by constricting the air flow in the system. A typical venturi tube has an inlet diameter which narrows down to a throat of a smaller diameter. The smaller cross-sectional area at the throat results in an increase in air velocity. A pressure tap monitors the pressure at the inlet, and a second pressure tap monitors the pressure within the throat. This pressure differential is then used to estimate the flow rate.
A measuring system similar to the venturi tube may have a limited aperture for constricting the air flow within a conduit. A pressure sensor is generally located upstream from the aperture and another pressure sensor is located downstream from the aperture. The pressure differential can be used to determine the approximate air flow through the aperture. This type of flow sensor typically creates a significant pressure drop in the air stream that can dramatically reduce system efficiency.
Another constriction-type air flow sensor utilizes a nozzle having a flared inlet shaped much like a trumpet bell, and which smoothly narrows to a cylindrical throat having limited diameter and terminating at an outlet. The cylindrical throat has a constant circular cross-sectional profile over an extended length. Pressure measurements are taken at holes distributed along a portion of the cylindrical length. Air flow can then be calculated based upon a pressure comparison with the region upstream of the nozzle.
In general, the accuracy of both the obstruction-type measuring systems and the constriction-type flow measuring systems is increased by increasing the flow resistance. Of course, such increased flow resistance adversely impacts on the overall system efficiency.